The design of spectacle lenses has several, often conflicting, considerations. These considerations can be grouped as optical, ergonomic, cosmetic, and economic. Presently available spectacle lens consists of a single refractive element which is a transparent homogeneous material bounded by two polished surfaces, usually in the form of a meniscus. Therefore, to design spectacle lenses that offer a suitable compromise in terms of the above considerations, the designer refers to two main elements: 1) the optical and physical properties of the lens material, which consist of the refractive index, the constringence (Abbe number), and the density and 2) the geometry of the surfaces that bound the lens.
Until recently, the lack of appropriate optical machinery restricted the milling and polishing of optical surfaces to spherical and toric shapes. With the advent of Computerized Numerical Control (CNC) machines, the manufacturing of complex optical surfaces, the so-called free-form surfaces, became feasible, creating new possibilities in spectacle lens design.
Also recently, plastic materials with relatively high refractive indexes and moderately low Abbe numbers were produced, making possible the designs of lighter, flatter (small plate-thickness) and thinner (small edge or center thickness) lenses, without sacrificing optical performance.
A shortcoming of astigmatic lenses made with spherical and toric surfaces is the inability to simultaneously correct the mean power error and the astigmatic error in the two main meridians of the lens, namely, the base curve meridian and the cross curve meridian. The use of aspherical surfaces, that is, of non-spherical surfaces of revolution, does not solve the problem.